The present invention relates to the field of semiconductor devices, and more particularly to a method for forming an electrical contact to a porous silicon structure.
The band structure for single crystal silicon exhibits a conduction band minimum which does not have the same crystal momentum as the valence band maximum, yielding an indirect gap. Therefore, in silicon, radiative recombination can only take place with the assistance of a photon, making such transitions inefficient. This characteristic has prevented silicon from being used as a solid state source of light, unlike group III-V semiconductors which have a direct gap at the center of the Brillouin zone. However, the discovery of photoluminescence in porous silicon has therefore generated a new optoelectronic material for study.
Porous silicon is conventionally formed using electrochemical etching of silicon substrates. In addition to silicon layers, other materials such as germanium, silicon-germanium alloys and the like may also be etched into the porous form. The substrate may be suitably patterned lithographically prior to the etch to define device structures or confine the region exposed to the etch solution. The typical emission spectrum of porous silicon is in the red, orange and yellow region (nominally 500 to 750 nm) although green and blue emissions have also been demonstrated. Blue shift of the peak emission wavelength has been shown by increased oxidation and etching of the porous silicon. However, the efficiency of porous silicon light-emitting diodes remains low due to the difficulty in making solid state contacts to a highly porous structure.
Reversible quenching of photoluminescence may be obtained from porous silicon layers fabricated in bulk silicon due to surface adsorbates. Reversible quenching refers to a decrease in the light emission from the porous silicon that returns to its previous state of emission in the absence of the surface adsorbate. The degree of quenching nominally scales with the solvent dipole moment. Reversible quenching of porous silicon structures demonstrates that light emitting porous silicon structures are chemically fragile, i.e. their light emitting properties may be changed when they are placed in contact with a chemical element or compound. Furthermore, quenching the light emitting property is not reversible when porous silicon is subjected to some solutions and chemical elements commonly used in semiconductor processing. In addition, heating porous silicon structures and/or devices to temperatures approaching 300.degree. C., and above, permanently destroys the light emitting (photonic) properties of porous silicon.
The light emitting mechanism of porous silicon is not fully understood. The scientific controversy surrounding the physical mechanism behind the light emission has not, however, hindered the ability to fabricate porous silicon layers and useful light emitting devices. The prior art uses evaporated or sputter-deposited layers of conductive materials such as semi-transparent gold or indium tin oxide (ITO) to make electrical contact to the porous silicon layers and device structures. These techniques are line-of-sight deposition techniques that can not fill voids in the porous silicon structure due, in part, to the large particle size and the directionality of the deposited material. The opacity of the silicon substrate used by the prior art adds an additional constraint because the electrical contacts must be optically transparent to allow emission of light from the porous silicon devices.
FIG. 1 schematically shows a prior art porous silicon region 10 formed in a silicon structure 12. The silicon structure 12 is covered by an electron beam sputtered layer of a conductive metal 20 such as indium-tin-oxide (typically 95% indium oxide, 5% tin oxide). Voids 30 formed in the silicon structure 12 are not efficiently filled which prevents effective electrical contact between the conductive metal 20 and the silicon structure 12.
Therefore, a need exists for a method for forming a low electrical resistance, high surface area contact to a mechanically, chemically and thermally fragile, porous semiconducting structure in a manner that preserves the photonic properties of the semiconductor.